Band-pass selector system



Oct. 8, 1940. J. F FARRINGTON BAND-PASS SELECTOR SYSTEM Filed April 27, 1938 FlG.l.

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IINVENTOR ATTORNEY 35 the input and output circuits, wherein each of the be varied by adjusting the value of a biasing po-. 35

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v 1 STAT P T N O ICE John F. Farrington, Flushing, N. Y., assignosto Hazeltine Corporation, a corporation or Dela-3 7 Application April 27, 1933, Serial No. 204,528

irciaims. (oi. 1791 171) This invention relates to band-pass selector a combination of directive and non-directive cou-' systems and, more particularly, to such systems plings. Such an arrangement is disclosed and in which the width of the frequency band transclaimed in United States Letters Patent No. mitted through the system is adjustable. While 2,033,330, granted March 10, 1936,on'the appli- 5 band-pass selector systems constructed in accation of Leslie F. Curtis. That patent discloses cordance with the present invention are of genan arrangement having inductive coupling beeral utility, they are particularly suitable for use tween two tuned circuits in the signal-translatin the intermediate-frequency channel of moduing channel of a signal receiver and alsohaving lated-carrier signal receivers of the superheteroan additional directive coupling in the backward m dyne type, i'orcontrolling the selectivity and fideldirection through a tube in the si s a 1. ity' of response of the receiver. a channel preceding the tuned circuits.

Band-pass selector systems of conventional 4 It is an object of the present invention, theredesign usually comprise a pair ofresonant cirfore, to provide animproved band-pass selector cuits tuned to the same or difierent frequencies system of simple arrangement inwhich the total and suitably coupled inductively, capacitively, or coupling between the input andoutput circuits u by a combination of these individual couplings. of the system is partially directive and partially The responsivenessof thistype oi selector system ond c and Which i Capable of uniformly is substantially constant over a band of ii'etranslating a desired band of frequencies with quencies in the'vicinity of the mean resonant Iresharp discrimination against all other frequenquency of the selector, while signal components a cies. a go of all other frequencies are sharply discriminated It is another object of the invention to provide against and areattenuated to a substantial dean p d band-D888 88160501 System which is gree by the selectorr lin general, the width of capable of amplifying in the coupling path the the frequency, band passed by such a system may signal components transmitted through the sysbe varied eitherby changing the coupling betem. V

tween the two circuits or by adjusting the reso- It is a further object of the invention to pro-' nant frequencies of the two circuits relative to vide an improved band-pass selector system of the 1 each other. The couplings referred to are nonp described which is readily adjustable to y directive in nature; that is, either circuit may be the widthof the frequency band transmitted by made the input circuitf and the other the output the system. V r 30 circuit without substantially afiecting the char- More particularly, it isan object of the invenacteri'stics of the system. Such'a' systemis to be tion to provide an improved band-pass selector contrasted with the type of selector utilizing a system of the type described in which the. width vacuum-tube coupling in each directionbetween OI t e eq y a Passed y the System y couplings is primarily unidirective. Such a systential applied to a control electrode of a vacuum tem isdescz ibed and claimed in the copendingaptube connected in the signal-translating channel plication of Harold M. Lewis, Serial No. 70,172, of the system at a point in the channel succeeding filed March 21, I936. a the coupled tuned circuits. t

40 Band-pass selectors wherein the nondirective In accordance with the invention, a signal-w 40 form of coupling is utilized are, in general, open translatingchannel comprises a band-pass selecto the criticism that only mechanical or relator system for passinga band of, frequencies. The tively comphcated nonmechanical expedients are selector includes input nd Output tuned circuits known for adjusting the width of the frequency resonant at frequencies within the 'band and band to be transmitted. Further, the convenmutual 'reactance means coupling'the circuits, 45 ticnal type of coupling referred to is inherently as well as unidirective coupling means-coupling incapable of producingamplificationof the transthe'circuits in the backward direction.- The unimitted signal :components. Band-pass selector directive coupling means comprises a vacuum circuits employing unidirective. couplings inthe tube having input and output electrodes and an forward and backward directions between tuned output circuit coupled to the output electrodes, 50 terminal circuitsfi however, require a separate while means are provided for-impressing at least vacuum tube,'or the equivalent, in each 01 the Iora portion of the voltage across one of the tuned ward and backward coupling paths. Band-pass circuits between the input electrodes. Means are" selectors are known in which the total coupling also provided for deriving a voltage from the outat between the tuned terminal circuits is provided by put circuit of the tube, having a phase substanu 2 tially independent of the impedance of the output circuit, and for coupling the derived voltage effectively in series in the other of the tuned ciriuits. The last-named coupling means are substantially less frequency-selective than the tuned circuits and co-operate with the tuned circuits to provide a coupling reaction between the tuned circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below the resonant frequency. To secure a unidirective form of coupling, an element having a directive transconductance, such as a vacuum tube, is thus included in the backward coupling I means. In the preferred embodiments of the invention, this vacuum tube is included in the signal-translating channel at a point succeeding said tuned circuits. With this form of coupling between the circuits, the backward coupling may be varied at will by varying the value of transconductance of the directive coupling element, for example, with a control grid-biasing potential, to adjust the band-pass characteristic sym metrically with respect to the mean resonant frequency of the system. The response characteristics obtained are somewhat like those procured by symmetrically detuning a pair of loosely coupled tuned circuits as described in the paper entitled High fidelity receivers with expanding selectors by Wheeler and Johnson, published in Proceedings of I. R. E., June, 1935, at pp. 594- 609. Further, in accordance with the invention, means may be provided whereby the grid-cathode capacitance of the vacuum tube comprising the directive coupling means may be held constant as the control grid-bias potential is changed to control the selective properties of the system. This prevents detuning of the output circuit of the selector and enables symmetrical expansion of the band-pass characteristic of the selector to be procured.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in connection with the accompanying drawing, in which Fig. 1 is a circuit diagram of an embodiment of the invention; Fig. 2 is a circuit diagram, partially schematic, of a radio broadcast receiver of the superheterodyne type employing a band-pass selector constructed in accordance with the invention; while Figs. 3-5, inclusive, illustrate different embodiments of the invention.

Referring now more particularly to Fig. 1 of the drawing, there is shown a band-pass selector comprising a tuned primary circuit coupled to the output electrodes of a vacuum tube in and a tuned secondary circuit coupled to the input electrodes of a second vacuum tube l. The primary circuit comprises series-connected inductance l2, resistor l3 representing the inherent resistance of inductance l2, and resistor l4 across which is connected the tuning condenser l5. The secondary circuit comprises series-connected inductance I 6 and resistor l1 representing the resistance of inductance 16, across which is connected a tuning condenser l8. Inductance I2 is inductively coupled to inductance l6, providing a mutual reactance means coupling the input and output tuned circuits. A load circuit (not shown) is provided for the output circuit of vacuum tube H and may be connected between the output terminals 2!. Input terminals 20 are connected to the input circuit of tube It.

In explaining the operation of the circuit of Fig. 1, it is assumed that both of the tuned circuits comprising the selector are resonant at the same frequency. Reference is made to the fact that, in the usual intermediate-frequency transformer having nondirective inductive coupling between the windings thereof, the primary current induces a voltage E12 in series with the secondary circuit. This secondary induced voltage causes a secondary current to flow which reacts on the primary circuit, inducing a counter Y voltage E21 in series with the primary circuit. This counter voltage has a marked effect on the magnitude and phase characteristics of the primary current as a function of frequency. The primary-current response curve exhibits a double peak at coefficients of coupling even less than optimum, because E21 represents a voltage that decreases rapidly on either side of the common resonant frequency of the primary and secondary circuits. This double-peaked primary current is masked in its effect on the secondary current at less than optimum coupling by the selectivity characteristics of the secondary circuit. However, at couplings greater than optimum, the secondary current characteristic also becomes double-peaked, although never to such a marked degree as the primary-current characteristic. When the circuits are overcoupled by mutual inductance, it is possible to procure a sharp overall response characteristic and wipe out the loss in transfer efficiency through the selector at the resonant frequency by introducing in series with the primary circuit a feed-back voltage of opposite phase to the counter voltage E21. The resistor M in the cathode circuit of tube ll of Fig. 1 provides such feed-back voltage when the mutual inductance between coils I2 and i6 is poled properly. This auxiliary feed-back voltage is proportional to the mutual conductance of amplifier tube ll. Consequently, by controlling the grid bias of this amplifier, the response characteristic of the selector may be varied from a condition of efficient signal transfer and high selectivity to one of lower signal transfer and broad selectivity. In the circuit of Fig. 1, amplifier tube ll succeeds selector l2, l5, I6, IS in the signal-translating channel of the system and is utilized to per- .form the dual function of providing the usual amplification and of providing the feed-back voltage in the primary circuit varying in accordance with its mutual conductance. Insofar as the band-pass characteristics of the selector are concerned, the circuit of Fig. 1, taking into account the transconductance of vacuum tube ll, gives response characteristics as if there were an effective coeflicient of coupling keff between windings l2 and I6 of A suitable variable selector design is obtained by making M suficiently large to obtain the desired maximum band width or peak separation when the transconductance of tube I l approaches zero and solving for R: by setting km=0. The resonance characteristic of the selector of the invention for km= is the same as would be obtained if the tuned circuits J and It," were in different stages of the system. The desired average power factor Po is obtained by selecting suitable values for coil resistances l3 and II.

It will be understood that the vacuum tube l I comprises a unidirective coupling means coupling the input andoutput tuned circuits in the backwarddirection and that the voltage derived from the ouput circuit of tube II, which is fed back to'the tuned circuit l2,l5, has a phase substantially independent of the impedance of the output circuit of tube Ii. It will also be understood thatthe directive coupling path comprising the transconductance of vacuum tube II and resistor I4 is substantially less frequency-selective than the tuned circuits l2," and I6," and that the directive coupling path co-operates with the tuned circuits to provide a coupling reaction therebetween which is regenerative at frequencies in the vicinity of the mean resonant frequency of the system and which is decreasingly regenerative at frequencies above and below said mean resonant frequency, in a manner more fully described in the above-mentioned Lewis application. v

Fig. 2 is a circuit diagram, partially schematic, of the improved band-pass selector system, embodied in a broadcast receiver of the superheterodyne type, for controlling the selectivity of the intermediate-frequency channel of the receiver. Briefly described, the receiver comprises a radiofrequency amplifier having input terminals connected to an antenna 3| and ground 32 and output terminals coupled to a frequency changer or oscillator-modulator 33; Connected in cascade with the output circuit of frequency changer unit 33, in the order named, are an intermediate-frequency amplifier comprising vacuum tube Iii, a variable-band-pass selector system and intermediate-frequency amplifier comprising inductances l2 and I6 and vacuum tube l I, a detector and automatic amplification control unit 34, an audio-frequency amplifier 35, and a sound reproducer 36. The bias potential derived from the automatic amplification control source 34 may be applied to one or more of the stages of radiofrequency amplifier 30, the modulator included in frequency changer unit 33 and oneor both of the intermediate-frequency amplifiers l0 and II.

Considering first the operation of the receiver as a whole, without regard to the details of the band-pass selector of the invention, per se, a desired received signal is selected and amplified in radio-frequency amplifier 30 and is converted to a modulated intermediate-frequency signal by frequency changer 33. The signal as thus converted is further amplified in intermediate-frequency amplifiers l0 and II and detected by detector 34, thereby producing the audio frequencies of modulation which are, in turn,,amplified by the audio-frequency amplifier 35 and reproduced by the sound reproducer 36. The amplification of the received signal is subject to automatic control by the control-bias potential derived from source 34 according to the manner well understood in the art.

Referring now more particularly to the details of the band-pass selector system constructed in accordance with this invention, the system comprises a band-pass selector and amplifier coupled between terminals 20 and terminals 2|. The band-pass selector of Fig. 2 is similar in.many respects to the band-pass selector of Fig. 1 and similar circuit elements have been given identical reference numerals. The band-pass selector of Fig. 2 differs from that of Fig.1 only in the connection of the return lead of the grid circuit of vacuum tube Ii which, in the circuit of Fig. 2, is connected to ground through a blocking condenser 31 rather than directly to the cathode of tube II as in the circuitof Fig. 1. A decoupling resistor 39 is provided in the output circuit of tube Ill.

The operation of the circuit of Fig. 2 is similar to that of Fig. 1 except that a desirable effect is secured, as is explained hereinafter in more detail, due to the fact that resistor I4 is included in the input circuit of tube ll.

The circuit of Fig. 3 is similar to the embodi- [ment of the invention disclosed in Fig. 2,- the primary diflerence being that the resistor I4 is included in the capacitive 'leg of tuned circuit 1 I2,l5 rather than in the inductive leg, as in Fig. 2.

The circuit of Fig. 3 gives the same performance as the circuit of Fig. 2 provided the polarity of the coupling between inductanoes l2 and I6 is positive rather than negative as in the case of the couplings between the corresponding inductances of Fig. 2.

In the circuits of Figs. 1, 2, and 3 the effect of feed-back resistance I 4 in lowering the effective Q (uL/r) of the primary circuit comprising inductance l2 and condenser l5 can be lessened to some extent by increasing the self-inductance of inductances l2 and I 6 as well as the ratio of the inductance of winding It with respect to that of winding l2 within the limits of practicability.

In the embodiment of the invention disclosed in Figs. 4 and 5, the effect of the cathode feed-back resistor of the control tube H can be made negligible by employing an impedance transformation. Referring particularly to the circuit of Fig. 4, the selector is generally similar to that of the preceding figures and similar circuit elements have been given identical reference numerals. The cathode feedback resistor included in the output-circuit of the control tube l I has been given the reference numeral 40 to indicate that it is of a different order of magnitude than the cathode-resistor utilized in the circuits of Figs. 1, 2, and 3. The voltage across the cathode-resistor 40 is coupled into the primary tuned circuit comprising condenser l5 and inductance I2 by means of an auxiliary inductance 4i connected across resistor 40 and inductively coupled to inductance 12. The circuit is so proportioned that wiJL=SNRk and woMa=SRk where Thecircuit of Fig. 5 is generally similar to the circuit of Fig. 4, the small capacitance 42 serving the same purpose as the mutual inductance between windings l2 and 41 of Fig.4. Both circuits serve to introduce an imp'edance transformation Ci=NCa where C1=sum of the capacitances of condensers I5 and 42. Cs=capacitance of condenser 42.

The value of N selected in either Fig. 4 or Fig. 5 has an identical effect on the performance of each circuit. In practice, NRk, which is the value of resistor 40 of Figs. 4 and 5, is ordinarily made large enough to serve as a self-bias resistor for tube II, which requires a resistance of around 250 to 350 ohms. Rx, which is the value of resistor It in Figs. 1, 2, and 3, is of the order of ohms, thus making N about 15. Therefore, the resistance coupled into the primary circuit through the mutual inductance between inductance I2 and ll of Fig. 4 or through the capacitive coupling of condenser 42 of Fig. 5 is only one to two ohms.

.When the grid lead of tube II of the above circult is returned to the cathode, as in the embodiment of Fig. 1, the performance of the system may, in certain applications, be impaired by the presence of inherent capacitance between the grid of tube H and ground. This capacitance causes an increase in the resonant frequency of the secondary circuit I6, I8 as the transconductance of control tube .II isdecreased. Inasmuch as any variation of'the control tube input capacitance with transconductance shifts the secondary resonant frequency in the same direction, the response curve may become asymmetrical at low values of transconductance. However, with the grid return lead connected to ground, as shown in the embodiments of Figs. 2, 3, 4, and 5, the circuit is stabilized even when the input capacitance of vacuum tube I I varies with variations of the transconductance of tube II. This is true because the grid and cathode circuits of vacuum tube II in these embodiments of the invention complete a compensating circuit. A resistance component and a reactance component are thus reflected into circuit I6, I8 due to feedback from the voltage developed across the resistor which is common to the input and output circuits of tube I I. The resistance component is negative. up of two terms, one term of which is independent of frequency but which increases as the value of transconductance gm of tube I I decreases, and the other term of which is a function of frequency. The circuit is so proportioned that the second term is appreciably smaller than the first term. This can be done by returning screen and suppressor grids of tube H to ground rather than to its cathode. The circuit is then proportioned so that the desired compensation is secured. In case over-compensation for input capacitance variation with transconductance results from a particular circuit design, the grid return lead may be connected to a tap on resistor It or resistor 40, as shown in Fig. 5, to obtain exact frequency compensation. In case under-compensation results from a particular circuit design, the condenser I8 alone may be returned to' a tap on resistor 40. It can be demonstrated mathmatically that the magnitude of the frequency shift, due to variation in transconductance of the control for a given value of cathode resistance, may be increased by directly inserting a portion of the cathode resistance in series with the capacitance branch I8 of the secondary circuit. As illustrative of a par- The capacitance component is made ticular embodiment of the invention, the following circuit constants are given for an embodiment of the invention of the type disclosed in Fig. 3 in which the mean resonant frequency of the tube I I 0.6 millihenrys As illustrative of another embodiment of the invention in a system having a mean frequency of 450 kilocycles, the following circuit constants 20 are given for an embodiment of the circuit of Fig. 5:

Tube III Type 6A8 4 Tube II Type 6K7 Inductance I2 0.52 millihenrys Q of primary circuit I2, I5 .115 Inductance Ii: 0.466 millihenrys Q of secondary circuit I6,

. I3 115 Resistor 4s 250 ohms 80 Condenser 62 30 micro-microfarads Inductance in the anode circuit of tube I I .0.6 millihenrys In each of the foregoing examples, the tube I 0 is specified as a pentagrid converter tube of the type 6A8 and an embodiment utilizing a tube of this type is suitable for use as an oscillatormodulator, for instance, as the oscillator-modulator 33 of the receiver of Fig. 2. However, tube III may be a conventional pentode, as illustrated in the drawing, if the tube is'to be utilized as an intermediate-frequency amplifier in a radio receiver, as shown in Fig. 2.

It will be understood that operating potentials, together with suitable blocking condensers to restrict the flow of direct currents to the tube circuits, are provided for the circuits of Figs. 1, 3, 4, and 5 in a conventional manner.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having input and output electrodes and an output circuit coupled to said output electrodes, means for impressing at least a portion of the voltage across one of said tuned circuits between said input electrodes, and means for deriving a voltage from the output circuit of said tube having a phase substantially independent of the impedance of said output circuit and 75 for coupling said derived voltage effectively in series in the other of said tuned circuits, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

2. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having a variable transconductance and having input and output electrodes and an output circuit coupled to said output electrodes,

means for impressing at least a portion of the voltage across one of said tuned circuits between said input electrodes, and meansfor deriving a voltage from the output circuit of said tube having a phase substantially independent of the impedance of said output circuit and for coupling said derived voltage effectively in series in the other of said tuned circuits, means for varying said transconductance to vary the effective cou- -pling between said circuits, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

3. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having input and output electrodes and an output circuit coupled to said output electrodes, means for impressing at least a portion of the voltage across said output tuned circuit between said input electrodes, and means for deriving a voltage from the output circuit of said tube having a phase substantially independent of the impedance of said output circuit and for coupling said derived voltage effectively in series in said input circuit, said coupling means being substantially less frequency-selective than. said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

4. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having input and output electrodes and an impedance coupled between said output electrodes, said input electrodes being effectively coupled directly across at least a portion of said output tuned circuit, means for deriving a voltagefrom the output circuit ofsaid tube having a phase substantially independent of said impedance and for eifectively coupling said voltage in series in said input tuned circuit, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and'below said resonant frequencies.

5. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction,-

means being substantially less frequency-selec-' tive than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the. vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

6. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said selector comprising input and output tuned circuits resonant within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a repeater in said signal channel having input and output electrodes, said input electrodes being coupled to said output circuit, an impedance in the load circuit of said vacuum tube, and a circuit for deriving from said load circuit a voltage having a phase substantially independent of said impedance and for coupling said voltage into said input circuit, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies i 7. A signal-translating channel comprising a band-pass selector system for passing aband of frequencies, said selectorcomprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum-tube repeater in said channel at a point succeeding said tuned circuits and having input and output electrodes, said input electrodes bein coupled to said output circuit, a cathoderesistor for said vacuum tube, and a circuit for coupling at least a portion of the voltage developed across said resistor to said input circuit, said coupling means being substantially less frequency-selective than said input and output circuits and cooperating with said circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

8. A signal-translating channel including a band-pass selector system for passing a band of frequencies, said selector system comprising input and output circuits resonant at frequencies within saidband, means comprising mutual reactance coupling said circuits,-unidirective coupling means coupling said circuits in the backward direction, said unidirective means comprising a vacuum-tube repeater in said channel having input electrodes coupled to said output circult, and a cathode-resistor for said vacuum tube, said resistor being effectively coupled in series in said input circuit, said coupling means being substantially j less frequency-selective than said input and output circuits and co-operating with said input and output circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

9. A signal-translating channel including a band-pass selector system for passing a band of frequencies, said system comprising input and output circuits resonant at frequencies within said band, said input circuit comprising a capacitance leg, means comprising positive mutual iiiductance coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum-tube repeater in said channel having input electrodes coupled to said output circuit, and a cathode-resistor for said vacuum tube, said resistor being efiectively coupled in series in said capacitance leg, said coupling means being substantially less frequency-selective than said input and output circuits and cooperating with said input and output circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.-

10. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said system comprising input and output circuits resonant at frequencies within said band, means comprising mutual reactance coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum-tube repeater having a variable transconductance in said channel at a point succeeding said tuned circuits and having input electrodes coupled to said output circuit, a cathode-biasing resistor for said vacuum tube, said resistor being effectively coupled in series in said input circuit and in the input circuit of said tube, and means for varying said transconductance, said coupling means being substantially less frequency-selective than said input and output circuits and cooperating with said input and output circuits to provide a coupling reaction between said cii cults which is regenerative at frequencies within the vicinity of said resonant frequencies andand decreasingly regenerative at frequencies above and below said resonant frequencies.

11. A signal-translating channel comprising a band-pass selector system forpassing a band of frequencies, said system comprising input and output circuits resonant at frequencies within said band, means comprising mutual reactance coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube in said channel having input electrodes coupled to said output circuit, a cathodebiasing resistor for said vacuum tube, and impedance transforming means for coupling at least a portion of the voltage across said resistor into said input circuit, said coupling means being substantially less frequency-selective than said input and output circuits and co-operating with said input and output circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

12. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said system comprising input and output circuits resonant at frequencies within said band, means comprising mutual reactance coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum-tube repeater in said channel having input electrodes coupled to said output circuit, a cathode-biasing resistor for said vacuum tube, and means comprising an inductance inductively coupled to said input circuit for coupling at least a portion of the voltage across said resistor into said input circuit, said coupling means being substantially less frequency-selective than said input and output tuned circuits and co-operating with said circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

13. A signal-translating channel comprising a band-pass selector system for passing a band of frequencies, said system comprising input and output circuits resonant at frequencies within said band, means comprising mutual reactance coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube in said channel having input electrodes coupled to said output circuit, a cathodebiasing resistor for said vacuum tube, and means comprising a condenser effectively coupled to said input circuit for coupling at least a portion of the voltage across said resistor to said input circuit, said coupling means being substantially less frequency-selective than said input and output circuits and co-operating with said circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

14. A band-pass selector system for passing a band of frequencies comprising input and output tuned circuits resonant at frequencies within said tions in the input capacitance of said vacuum tube with variations in its transconductance, said coupling means being substantially less frequency-selective than said tuned circuits and co-opcrating with said tuned circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

15. A band-pass selector system for passing a band of frequencies comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance coupling means coupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having a variable transconductance and having input electrodes coupled to said output circuit and output electrodes coupled to said input circuit, a cathode-biasing resistor for said vacuum tube, means for varying said transconductance to vary the effective coupling between said circuits, means comprising said cathode-resistor for preventing a variation in the input capacitance of said tube with variations in said transconductance, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said circuits which is regenerative at frequencies within the vicinity of said resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

16. A band-pass selector system for passing a band of'frequencies comprising input and output tuned circuits resonant at frequencies within said band, mutual reactance means coupling said circuits, unidirective coupling means coupling saidcircuits inthe backward direction, said unidirective coupling means comprising a vacuum tube having a variable transconductance and having input electrodes coupled to-said output circuit and output electrodes coupled to said input circuit, ,a cathode-biasing resistor for said vacuum tube, means for varying said transconductance to vary the effective coupling between said circuits, means comprising an adjustable portion'of said cathode-resistor for preventing a variation in the input capacitance of said tube with variations in said transconductance, said coupling means being substantially less frequency-seleqtive than said tuned circuits and cooperating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

17. A band-pass selector system for passing a band of frequencies comprising input and output tuned circuits resonant at frequencies within said band, mutualreactance coupling means ecupling said circuits, unidirective coupling means coupling said circuits in the backward direction, said unidirective coupling means comprising a vacuum tube having a variable transconductance and having input electrodes coupled to said output circuit and output electrodes coupled to said input circuit, a cathode-biasing resistor for said vacuum tube, means for varying said transconductance to vary the eiiective coupling between said circuits, means comprising said cathode-resistor for effecting a variation in the input capacitance of said tube with variations in said transconductance, the value of said resistor being a, compromise between the value required for cathode biasing, band width adjusting and eliminating variation in the input capacitance of said tube with transconductance, said coupling means being substantially less frequency-selective than said tuned circuits and co-operating with said tuned circuits to provide a coupling reaction between said tuned circuits which is regenerative at frequencies within the vicinity of the resonant frequencies and decreasingly regenerative at frequencies above and below said resonant frequencies.

JOHN F. FARRINGTON. 

